This invention relates to a device for dispersing flocs in a papermaking process and, more particularly, to a device for dewatering of the sheet in a papermaking process while simultaneously dispersing the flocs.
The sheet forming process is composed of three hydrodynamic processes. These processes are drainage, oriented shear, and turbulence. Drainage is characterized as perpendicular flow through a screen (forming fabric) that varies with time. Oriented shear is fluid shear flow having a distinct pattern or direction in the undrained fiber suspension. Turbulence is the random fluctuation in fluid flow sheet forming process, papermakers control drainage, oriented shear and turbulence to alter sheet properties.
The most difficult hydrodynamic process to control is oriented shear, specifically cross-machine direction shear, although oriented shear can exist in the machine direction as well as in the cross-machine direction. Machine direction shear is generated generally by a velocity difference between the headbox jet and the forming fabric (forming wire) speeds. Cross-machine shear is generally generated by machine-direction ridges. That is, during the sheet forming process, a phase shift occurs as these ridges encounter the vacuum pulse of drainage elements. When this occurs, the hills become valleys and vice versa. The movement of stock downward from a ridge into a valley creates the cross-machine shear. One such device for controlling cross-machine shear is disclosed in U.S. Pat. No. 4,532,009. This machine discloses the provision generally of a plurality of notches in the leading or upstream edge of the forming board elements. The center line of these notches extends parallel to the direction of movement of the forming fabric or forming wire. This structure does not effect a satisfactory mixing of the drained water back into the fiber slurry due, it is believed, to the alignment of the longitudinal axes of the notches in a direction parallel to the direction of travel of the forming fabric or forming wire.
Accordingly, it is an object of this invention to provide a hydrofoil blade having elements which generate crosswise streams in the stock on a forming fabric of a papermaking machine.
Another object is to provide a hydrofoil blade, as aforesaid, which has a relatively simple shape and, therefore, is easy to manufacture.
The objects and purposes of the invention are met by providing a hydrofoil blade having a top surface with a leading edge in contact with a forming fabric and a trailing edge, and a leading side surface disposed transversely to the direction of movement of said fabric upstream of and generally at an acute angle to the top surface thereby draining water from the stock. A plurality of slots extend from the leading side surface to a point between the leading and trailing edges on the top surface to force a portion of the drained water back into the stock, each of the plurality of slots having parallel side walls and a bottom wall of varying depth. The side walls of the slots extend generally perpendicular to the bottom wall and to the top surface of the hydrofoil blade, each of the slots being of a uniform width and having a longitudinal axis that extends at an angle to the leading edge so that the drained water will travel laterally of the longitudinal direction of movement of the fabric into the stock on the fabric, whereby any flocs formed in the stock are dispersed in the transversal shear generated by the drained water.